Toughening of thermosets

ABSTRACT

A method of manufacturing thermosets such as epoxy resins includes adding expandable hollow microspheres, which expand with temperature as shown in the accompanying graph, to the base thermoset components in the liquid phase and applying heat treatment to the mixture so formed causing the micropsheres to expand during or after curing of the thermoset. This results in a toughening mechanism caused by compressive residual stress around the microspheres which significantly increases the specific fracture energy of the epoxy resin.

FIELD OF THE INVENTION

This invention relates to the toughening of thermosets and has beendevised particularly though not solely for application to epoxy resins.

BACKGROUND OF THE INVENTION

Thermosets, such as epoxy resins, are brittle compared to thermoplasticsbecause of their cross-linked molecular structures. Attempts have beenmade in the past to increase the toughness of thermosets, particularlyepoxies, using the addition of liquid rubber or hard particles.

Liquid rubber has been successfully used as a toughening agent toincrease the specific fracture energy. Its toughening mechanisms includebridging, cavitation, crack pinning, crack blunting etc. In addition toliquid rubber, other toughening agents such as hard particles, hardhollow microspheres and coreshell rubber have also been used.

Another development in this area is an attempt to toughen thermoplasticsusing a similar method to the one used for ceramics in which fracturetoughness increase was achieved by a volume dilatation in the vicinityof crack tip resulted from tetragonal to monoclinic phasetransformation.

The present invention results from the realisation that thepre-stressing of the epoxy matrix, and the creation of residualcompressive stress may be performed using expandable hollow microspheresand heat treatment to achieve a similar effect to that of the phasetransformation of ceramics.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method of manufacturing athermoset including the steps of adding expandable hollow microspheresto at least one of the base thermoset components, and applying heattreatment to the partially or fully cured thermoset, causing theexpandable microspheres to expand during, or after, curing of thethermoset.

Preferably the thermoset comprises an epoxy resin and the expandablehollow microspheres are added to the epoxy component.

Preferably the mixture of epoxy component and expandable hollowmicrospheres are heated for easy mixing before adding a curing agent.

Preferably the mixture is allowed to cool before adding the curingagent.

Preferably the mixture is stirred after adding the curing agent, andpoured into a mould for curing.

Preferably the expandable hollow microspheres consist of a co-polymershell and gas.

Preferably the cured epoxy system is heated to create compressiveresidual stress around the expandable hollow microspheres.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms that may fall within its scope, onepreferred form of the invention will now be described by way of exampleonly, with reference to the accompanying drawing which is a graph of thevolume incremental expansion of expandable hollow microspheres due toheating.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In the preferred form of the invention, a thermoset in the form of anepoxy resin is toughened by the addition of expandable hollowmicrospheres, but it will be appreciated that this method can be appliedto any other form of thermoset material.

A conventional resin system such as the resin system consisted of WestSystem Epoxy 105 (a blend of Bisphenol A and Bisphenol F) and WestSystem Slow Hardener 206 (a blend of aliphatic amines and aliphaticamine adducts based on diethylene triamine and triethylenetetramine) ascuring agent, has a typical density of 1.1 for the resin system.

The system can be modified by the addition of expandable hollowmicrospheres as demonstrated in the following example.

Modifier used was hollow microspheres (EXPANCEL, 551 DU40, Akzo Nobel)which consist of co-polymer shell and gas. The chemical structure of themicrospheres was found to be (C₅H₈O₂—C₃H₃N—C₂H₂Cl₂)_(x) using a PerkinElmer Fourier Transform Infra Red Spectrometer, Paragon 1000. Themicrospheres expand when heated. By way of example, microspheres wereput in a 100 ml measuring cylinder and tapped for 5 minutes and thenplaced in an oven pre-heated to 70° C. Further heating of the ovenfollowed every 5 to 6 minutes for an increment of 10° C. until itreached 200° C. Resulting volume expansion of the microspheres measuredas a function of temperature is shown in the accompanying diagram.

In a typical method of manufacture of toughened epoxy, a quantity ofexpandable hollow microspheres are added to the epoxy component andstirred for approximately ten minutes. The mixture is then heated toapproximately 85° C. for thirty minutes to reduce the viscosity for easystirring before being allowed to cool gradually e.g. in a water bath forabout half an hour. The curing agent is then added and stirred for fiveminutes. The mixture can then be poured into a casting mould and left tocure at room temperature. Then, the moulded epoxy is heated at 135° C.for one hour and allowed to cool down to room temperature. Furtheradditional two hour heating is followed at 135° C.

Comparative Testing

Four different types of specimens were prepared viz pure epoxy (PE),heated pure epoxy (HPE), micro-sphere modified epoxy without heattreatment (ME), and micro-sphere modified epoxy with heat treatment(MEH). The HPE was to check any change in properties of PE due toheating which was also applied to MEH. The ME was to be used incomparison with MEH for checking any heat treatment effect on MEH.

The mixing of epoxy and curing agent was conducted by stirring for fiveminutes. The stoichiometric amount of curing agent was 17 phr (byweight) for all specimens. The mixture was poured into an aluminiummould with 6 mm thick cavity and left for curing at room temperature atleast for one day. Temperature rise in the mould due to exothermicreaction was monitored using a thermocouple and found to be about 8° C.which does not much effect the expansion of expandable hollowmicro-spheres (see accompanying drawing).

For both ME and MEH, a 10 phr of 551 DU was added to epoxy and stirredfor about 10 minutes. This mixture was heated to approximately 85° C.for 30 minutes to reduce the viscosity for easy stirring and thenallowed to cool gradually in a water bath for about half-hour. Thehardener was then added and stirred for 5 minutes. The casting wasconducted in the same way as for PE and HPE.

The heat treatment was conducted for HPE and MEH. Both HPE and MEH washeated in an oven at 135° C. for one hour and then allowed to cool downto room temperature. Further two hour heating was conducted at the sametemperature for 2 hours.

Results of the four different types of specimens for fracture toughness,flexural elastic modulus and flexural strength are listed in Table 1.Elastic modulus of PE appears to be not much affected by heat treatmentand modification with hollow microspheres. Also its flexural strengthappears to decrease due to modification without heat treatment but to alesser extent when the modification is accompanied by the heattreatment. Fracture toughness, however, increases due to bothmodification and heat treatment. The increase of PE in fracturetoughness after heating may be due to post-curing effect, althoughfracture surface features of HPE appeared to be different from that ofPE.

Microscopic work was conducted to identify toughening mechanisms of MEand MEH. During preliminary observation of SEM images, some differencesbetween MEH and ME were found. Some debonding between microspheres andmatrix was found in ME, which is different from that of hollow latexparticle with a styrene-acrylic shell, and some microspheres were pulledout. In contrast, MEH did not display any gaps between microspheres andmatrix, and cracking passed through the microspheres without anypull-out of microspheres.

When MEH is heated, it is found that microspheres naturally expandagainst the matrix resulting in both matrix, and subsequentlymicrospheres would permanently deform if deformation is sufficientlyhigh. Consequently, residual compressive stresses around microspheresare created when cooled down. In order to confirm the residualcompressive stress in the matrix, a thin section of MEH was examined andthe residual stress was indeed found to exist around microspheres. Thefringe patterns are observed around microspheres as evidence of theresidual stress.

The effect of compressive stress in the vicinity of a crack tip is wellknown. In the case of ceramics toughening, the compressive stress isproduced by localised volume dilation in the vicinity of crack caused byphase transformation. In the case of MEH, however, the compressivestress is distributed around microspheres throughout the whole specimen.Contribution of each microsphere to the toughness increase depends onits location—the closer to the crack tip, the more contribution.

Further to check if the residual stress in MEH is due solely to heattreatment, a thin section of ME was also examined and residual stressaround microspheres was found as well. An epoxy shrinks during curingand thus it seems that the residual stress was caused by shrinkage inthis case since it was not subjected to heat treatment. Accordingly, itappears that toughness increase in both ME and MEH were affected by thecompressive residual stress. Quantification of the residual stress wasnot attempted, but the residual stress in MEH should be higher than inME because the residual stress due to the epoxy shrinkage applies toboth MEH and ME, and hence the residual stress in MEH would be theresult of additional contribution of matrix shrinkage. Also, it had beenobserved using photo-elastic photos that there is generally much higherresidual stress in MEH than in ME.

No gaps between microspheres and matrix in the vicinity of the crack tipwere found in fast cracking regions for both ME and MEH. This suggeststhe gaps of ME are due to the cavitation when the crack slowlypropagates. However, there were no gaps in MEH.

TABLE 1 Mechanical properties of epoxies prepared. K_(1C) (MPa G_(1C)σ_(y) E Materials m^(1/2)) (kJ m⁻²) (MPa) (GPa) PE 0.65 0.30 130.65 1.42HPE 0.98 0.64 131.04 1.50 ME 1.30 1.06 67.16 1.60 MEH 1.46 1.57 103.621.36

It has thus been found that epoxy resins toughened in this manner by theaddition of expandable hollow microspheres and subsequently heatedexhibit increased fracture toughness compared with simple epoxy resinsof similar construction. It has been found that the heat treatmentimproves interfacial bonding between microspheres and the matrix.Compressive residual stress around microspheres, which may beresponsible for the major toughening mechanism, is successfully createdby expandable hollow microspheres with the heat treatment. It has alsobeen found that specific fracture energy of epoxy can be increased about13 times by this method.

1. A method of manufacturing a thermoset including the sequential stepsof: providing a curable mixture including a liquid base thermosetcomponent, a curing agent and expandable hollow microspheres; allowingthe curable mixture to cure, wherein a matrix forms around theexpandable hollow microspheres and the expandable hollow microspheresessentially do not expand; heating the cured mixture so that theexpandable hollow microspheres expand against the matrix causingpermanent deformation in the matrix around the microspheres; and coolingthe cured mixture such that compressive residual stress is created inthe matrix around the microspheres.
 2. A method as claimed in claim 1wherein the mixture comprises an epoxy resin.
 3. A method as claimed inclaim 1 wherein said providing includes adding the curing agent to aninitial mixture of the liquid base thermoset component and theexpandable hollow microspheres.
 4. A method as claimed in claim 3wherein said initial mixture is heated before adding the curing agent.5. A method as claimed in claim 4 wherein said heated initial mixture isallowed to cool before adding the curing agent.
 6. A method as claimedin claim 1 further including the step of pouring the mixture into amould.
 7. A method as claimed in claim 1 wherein the expandable hollowmicrospheres comprise a co-polymer shell and gas.